Arrays, iterables, covariance and Query Buses

[devnix]

Following the conversation I started in #5518 I provide the following example:
https://phpstan.org/r/1695407d-224f-4e63-8976-e0b48f4de2f4

Besides I think I understand covariance, contravariance and invariance, I'm not fully understanding the problem here. I'm not sure that I want QueryInterface to be covariant, and looks like my specific problem is solved by typing a non-mixed key and value in the iterable, so now I want to try to understand what can be happening with iterables.

The two main things I don't understand is this difference between array and iterable:

/**
 * @implements QueryInterface<array<int>>
 */
class ShouldNotFailArrayMixedValueQuery implements QueryInterface
{
}

/**
 * @implements QueryInterface<iterable<int>>
 */
class ShouldNotFailIterableMixedValueQuery implements QueryInterface // but fails
{
}

and why I have errors like Parameter #1 $query of method QueryBus::handle() expects QueryInterface<array<int, mixed>>, ShouldFailArrayMixedValueQuery given. if I'm passing ShouldFailArrayMixedValueQuery, which is exactly a QueryInterface<array<int, mixed>>.

This triggers in me a cognitive dissonance. Probably it is right and I'm just missing the point, but right now from my POV it feels like some kind of inconsistency.

[devnix]

Here I see too differences between how keys are inferred in different ways: https://phpstan.org/r/8e09ae8b-69c8-4dc2-8e59-4a8a61568efb

[ondrejmirtes]

With @template-covariant all of the examples passes: https://phpstan.org/r/13b4fc94-dceb-40ca-bd2f-7a7843def23e

In fact, I don't see why any of them should fail, can you explain please?

See also: https://phpstan.org/blog/whats-up-with-template-covariant

[devnix]

I did @ondrejmirtes, like a hundred times before writing the first post and again before my previous comment :(

If I'm just not understanding it, just don't worry, I've got a workaround with iterable<array-key, int> because it's in fact what I wanted to return, but I don't see why array<int> would work and iterable<int> not, or why a method expecting a QueryInterface<iterable<int>> is rejecting a QueryInterface<iterable<int>>, and I feel like it's a problem related with those non-empty-string and positive-int (being muted too by the @template-covariant tag).

If I'm just wrong please excuse me! For sure I don't have that much experience with generics and probably I'm wrong, I just wanted to make sure there is not the same bug.

Thank you for your attention, and thank you and the PHPStan contributors for your hard work! 🦾

[ondrejmirtes]

ShouldNotFailIterableMixedValueQuery is a subtype of QueryInterface<iterable<int>> (it's covariant, not invariant). That's why you need @template-covariant.

[VincentLanglet]

In fact, I don't see why any of them should fail, can you explain please?

I don't see a reason for having an error in https://phpstan.org/r/6cd47514-1fe4-40ad-969e-6b74d7336487, but still PHPStan are reporting errors, is it a bug ?

[devnix]

After following that a bunch of bugs related to generics were closed I've taken a revision to this, and I still think it's some kind of bug:

This is a new example, working as expected in Psalm and giving an error in PHPStan:
https://psalm.dev/r/b2b764f7b4
https://phpstan.org/r/f0fd711a-7701-4583-a564-0ad89ef9dc82

Happy holidays!

[ondrejmirtes]

@devnix This is reported correctly as TestQuery is subtype of Query<iterable<int>>.

Explained in this article: https://phpstan.org/blog/whats-up-with-template-covariant

You can fix this with @template-covariant, or call-site variance: https://phpstan.org/blog/guide-to-call-site-generic-variance